Dart knowledge: Optional, required parameters

• Optional parameter required parameters
  if required and optional parameters contains parameters required parameters before the parameter list, after the optional parameters.
  Optional parameters can have a default value, the default value is used when the caller does not specify a value. This and kotlin like syntax.
  Specific optional parameters can be divided into:
  1.Optional the named Parameters (optional named parameter)
  2.Optional Positional Parameters (optional position parameter)

• Source: https://www.jianshu.com/p/152c9c01ea9a